Technical Field
The present invention relates to optical switching and, more particularly, to optical switches that exhibit low crosstalk between outputs.
Description of the Related Art
Integrated 2×2 Mach-Zehnder interferometers can be used as basic elements of large transparent optical switches. An input signal is split in a 3-dB input coupler along two branches and the branches are subsequently recombined at a 3-dB output coupler. A phase controller is used on each branch to control how the signals on the branches are recombined, resulting in an output along either the first or the second branch of the output coupler. When the phase difference between the two branches is (2n+1)π, the first branch is selected, and when the phase difference is 2nπ, the second branch is selected.
In reality, however, there is usually some light leakage at the non-selected output that can be due to power imbalance in the interferometer, phase errors (where the phase controllers do not produce, for example, perfect integer multiples of pi), and imperfect couplers. This crosstalk puts a limit on the effectiveness of the switch, as less than full power is transmitted along the selected output and a potentially significant signal is present on the non-selected output.
One existing solution is to use a thermo-optic phase shifter on one branch that has a phase range from zero to pi. This can achieve a low crosstalk, but switching speeds are on a millisecond-scale, whereas optical switching networks could benefit from nanosecond-scale switching.
Electro-optical phase shifters using carrier injection in a pin diode, where a phase shift is obtained by modulating the carrier density. This can achieve nanosecond-scale switching rates, but carrier injection creates optical losses due to free-carrier absorption. A power imbalance results, affecting the crosstalk. Such a system produces, at best, −20 dB of isolation. This result can be improved somewhat by using push-pull drive configuration, with one phase shifter on each branch having a phase range of zero to one-half pi, but this produces only a moderate benefit and an isolation of about −25 dB.
As such, no existing solution can produce nanosecond-scale switching and a suitably low crosstalk.